This invention relates to a semiconductor device comprising a power transistor and a power semiconductor diode disposed on a common semiconductor substrate.
A conventional semiconductor device of the type referred to has comprised an N.sup.+ type semiconductor substrate serving as a collector layer of a power transistor, a first N.sup.- type semiconductor layer and a second N.sup.- type semiconductor layer superposed on one another in the named order. Then, a P.sup.+ type buried anode layer has been disposed on a predetermined portion of the surface of the first semiconductor layer contacting the second semiconductor layer and a P.sup.+ type anode wall has extended from a peripheral portion of the P.sup.+ type buried anode layer and reached the surface of the second semiconductor layer. Then, an N.sup.+ type cathode layer has been selectively disposed on the surface of that portion of the second semiconductor layer encircled with the anode wall resulting in the completion of a power semiconductor diode.
On the other hand, the power transistor has been produced by extending a P.sup.+ type base layer throughout the thickness of the second semiconductor layer on that portion spaced from the anode wall and then selectively disposing an N.sup.+ type emitter layer on the surface of the base layer.
In conventional semiconductor devices such as described above, the power diode has been low in both its withstanding voltage and dielectric strength because that portion of the second N.sup.- type semiconductor layer disposed between the cathode layer and the buried anode layer or its N.sup.- type cathode layer is thin, resulting in the punch-through effect occurring easily. In order to increase the punch-through voltage, the thickness of the second semiconductor layer might be increased. This has caused an increase in crystal defects therein, resulting in the deterioration of the characteristics of the power transistor and diode. Also, this increase in thickness has resulted in the power transistor having a collector body resistance which is increased beyond its required magnitude which causes a deterioration in the characteristics thereof.
Accordingly, it is an object of the present invention to provide a new and improved semiconductor device having a power diode with both an increased withstanding voltage and an increased dielectric strength, the power semiconductor diode being disposed on the same semiconductor substrate as a power transistor without deteriorating the characteristics of the power transistor.